Well, if I wanted to be an asshole...
The wood you get from trees, you know the woody stuff, that works great...
But if you can get pressure treated 4x's, as said already. The 2x4s work fine too but the bigger board wont warp and shit as much, and either way at least spray a coat of something on there, it wont rot if its treated but I've had screws rip out and shit.